A Ka-band Linearizer for TWTAs

A predistortion-type linearizer designed to be installed within a high power traveling-wave tube amplifier to provide improved carrier-to-intermodulation performance and near-constant phase transfer characteristics

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Even though traveling-wave tube amplifiers (TWTA) offer the best high power amplifier performance at high frequencies in terms of power, efficiency, size and cost, they lag behind solid-state power amplifiers in terms of linearity. Thus, some type of linearizer must be incorporated to improve the TWTA's linearity, particularly in light of today's demanding multicarrier transmit environments that use advanced digital modulation techniques and schemes for data compression. These techniques require intermodulation products and spectral regrowth to reach levels that only can be achieved with sophisticated linearization schemes.

Over the years, many linearization techniques have been developed that have their own set of difficulties and limitations. The predistortion linearizer has proven to be the most versatile of these methods. This technique is independent of input signal type and has better wideband performance than other available methods.

In the predistortion linearizer, a nonlinear transfer function is created that complements the amplifier's amplitude and phase nonlinearities. This transfer function then is used to predistort the incoming signal such that it is restored to its original waveform after passing through both the nonlinear predistorter and the amplifier. The linearizer's gain increases as the amplifier's gain decreases while the phase shift introduced by the linearizer is equal and opposite to that of the amplifier. Hence, both the gain and phase shift of the linearized amplifier are corrected.

The BAFL series linearizers are predistortion-type units designed to be installed within a high powerTWTA. The resulting combination provides improved carrier-to-intermodulation (C/I) performance and near-constant phase transfer characteristics, as shown in Figure 1 .

The model BAFL-28000G.d Ka-band linearizer covers the 26 to 31 GHz frequency range and typically is specified over a 1 GHz frequency bandwidth. This particular model is designed for use with TWT or klystron amplifiers. (An additional model is available for use with solid-state power amplifiers.) The linearizer utilizes the latest MMIC components and provides a greater than 10 dB reduction in distortion products for multicarrier traffic. The BAFL series linearizers require a minimum of adjustment, have a low failure rate and are highly stable over time and temperature variations.

Linearizer Performance

The major performance features of the BAFL-28000G.d linearizer are listed in Table 1 . Some of these parameters cannot be measured for the linearizer alone and, therefore, were determined using the characteristics of a typical TWTA. Figure 2 shows the power output and gain characteristics of a TWTA that suffers from nonlinear performance compared to a similar amplifier that has been corrected using predistortion. The output power vs. input power characteristic of the unlinearized TWTA rolls off as the amplifier approaches saturation and, as a result, a rolloff in gain occurs.

The predistorter's function is to introduce a variable loss before the amplifier such that the net gain is as shown for the linearized amplifier. Note that the difference in gain for the constant portion of the two gain curves is equal to the vertical separation of the Pin/Pout curves in decibels. The loss introduced by the nonlinear element is recovered by adding a linear gain block after the element. A similar correction occurs in the amplifier's phase characteristics.

Product Description

The linearizer's input port is connected through a voltage-variable attenuator followed by a low noise MMIC amplifier. This attenuator/amplifier sets the system gain and provides the level required by the linearizer. The linearizer is located after the MMIC amplifier followed by a second MMIC amplifier and voltage-variable attenuator. This additional amplifier compensates for the loss introduced by the linearizer module.

The BAFL-28000G.d linearizer is supplied in a compact 3.95" x 2.25" x 0.85" housing and equipped with K-type female RF input and output connectors. Connections for power and control are provided via a DB15P multipin connector. The unit is powered by +15 V DC and requires approximately 300 mA. A simplified block diagram of the linearizer is shown in Figure 3 .

Linearizer Adjustment

Alignment is accomplished digitally using an MS-DOS PC. A control program is supplied with the linearizer that controls the TWTA and stores the uncompensated gain's phase and magnitude (GPM) data at a particular temperature. Once the raw data are recorded, the linearizer's internal central processing unit (CPU) generates the compensation offsets to correct the TWTA's characteristics using an internal temperature-compensation program that corrects to the new GPM values. The CPU compares the GPM values to the new values in a look-up table, considers the present temperature and generates a set of offset values that are added to the values in the look-up table to maintain the new GPM values over temperature. The look-up table is preprogrammed to maintain the linearizer characteristic constant over a -50° to +85°C temperature range.

The BAFL linearizer is designed to be inserted into the system directly in front of the TWTA (or klystron). The addition of the linearizer has almost no effect on system performance except for the obvious improvement in amplifier linearity.

Alignment can be accomplished using one of two methods. The first method requires a spectrum analyzer, two-tone test generator and power meter. System performance is adjusted to maximize the C/I ratio for both magnitude and phase correction. The second method requires a vector network analyzer with power sweep capability. In this case, the analyzer is set to simultaneously display relative output power and either S21 log magnitude (gain) or phase with input power level. The linearizer then is adjusted to flatten the gain and phase curves while maintaining the TWTA's saturation point constant. Once the linearizer has been adjusted for optimum two-carrier C/I performance, it should provide good operation in a multicarrier environment.